Labels - Damp cloth first, if that didn't work, a damp Q-tip. works better for things like permanent marker or small lines. depending on the label, I might use a little alcohol or mild soap in the water. sometimes I've used goo gone, had good and bad results with it so be careful.
Shells - Goo Gone for stickers, then a damp cloth to clean other dirt and grime. really tough permanent marker stains I use Mr. Clean Magic Eraser but be warned that rubbing too much with it can smooth out a surface, which is bad for say a NES or 2600/7800 game as the plastic is bumpy. Mostly I use it ONLY as a last resort.
Contacts - Alcohol and a Q-tip. dip one side of the q-tip in alcohol and rub both sides of the contacts (contacts facing down so extra drip fall instead of going in the cartridge) flip the q-tip and gently dab the contacts to dry. wait a few minutes before testing cartridge.